Japanese Utility Model Patent Publication No. 6-67412 discloses a technique of reducing the strength of a mast of a stacker crane from a lower position to an upper position. The inventors studied to achieve weight reduction of the stacker crane, and to realize structure which makes it possible to reduce the weight while supporting the horizontal stress applied to the mast. As a result, the inventors arrived at the present invention.
An object of the present invention is to reduce the weight of a mast of a stacker crane.
Another object of the present invention is to provide a guide for elevation and lowering of an elevation frame using a mast formed by joining parts having different cross sections.
According to the present invention, a stacker crane includes lower and upper vehicles each having a travel motor, and a mast having nonuniform strength along a height direction. The mast is provided between the lower vehicle and the upper vehicle. The mast includes upper and lower end portions, and a central portion.
The central portion of the mast has low strength in comparison with both of the upper and lower end portions of the mast.
Preferably, the nonuniform strength of the mast is determined as:
the upper and lower end portions>the central portion in the height direction>an intermediate portion between the lower end portion and the central portion, and an intermediate portion between the upper end portion and the central portion.
Further, preferably, a surface of the mast facing an elevation frame from the lower end portion to the upper end portion is placed on a same plane, and at least the intermediate portions are made up of a plurality of hollow pipes to place both side surfaces of the mast from the lower end portion to the upper end portion, as viewed from the elevation frame, on the same planes, respectively.
In the present invention, a stacker crane includes lower and upper vehicles each having a travel motor, and a mast having nonuniform strength along a height direction. The mast is provided between the lower vehicle and the upper vehicle. The mast includes upper and lower end portions, and a central portion.
The central portion of the mast is formed to have low strength in comparison with both of the upper and lower end portions of the mast. In this manner, the weight of the mast is reduced.
In the present invention, the mast has the nonuniform strength in the height direction in correspondence with the stress distribution applied to the mast. In this structure, weight reduction of the mast is achieved, and the stacker crane can travel at high speed easily. Further, in the case where the surface of the mast facing the elevation frame is placed on the same plane from the lower end portion to the upper end portion, while the strength of the mast changes in the height direction, the elevation frame can be guided easily. Further, in the case where the narrow intermediate portions of the mast are made up of a plurality of hollow pipes, both side surfaces of the mast are aligned on the same planes from lower end portion to the upper end portion, respectively, to guide the elevation frame along the side surfaces of the mast easily.